Two GTPases, Ffh and Ftsy, interact during co-translational protein targeting in bacteria. The proteins, one a component of a ribonucleoprotein homologous to the signal recognition particle (SRP) of eukaryotes, and the other its membrane associated receptor, share a common two domain structure which comprises the N- and GTPase domains (N and G). We are investigating the structural roles of these proteins and their interactions with the signal sequence, the ribosome and with each other using X-ray crystallographic methods. Throughout these studies, we use the Computer Graphics Laboratory and the program MidasPlus to visualize small molecule ligands, electron density and atomic resolution protein structures determined by X-ray crystallography.